Issue a warning if "-r ." is used with two working directory parents.
Rationale for not aborting instead:
The first parent is usually more important as it is the local branch
during a merge and commands like 'hg diff' and 'hg diff -r.' behave still
identically (except for the warning of course).
Added a test for log -r. with one and two parents.
pulling from ../a
searching for changes
abort: repository is unrelated
pulling from ../a
searching for changes
warning: repository is unrelated
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
changeset: 1:bdcee5d51fa6
tag: tip
parent: -1:000000000000
user: a
date: Mon Jan 12 13:46:40 1970 +0000
summary: a
changeset: 0:f155ba1aa5ba
user: b
date: Mon Jan 12 13:46:40 1970 +0000
summary: b